diff --git a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt
index afbeaebbd33..8b8f00c7975 100644
--- a/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt
+++ b/compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt
@@ -879,6 +879,7 @@ object DIAGNOSTICS_LIST : DiagnosticList("FirErrors") {
         val PROPERTY_MUST_HAVE_SETTER by error<KtProperty>()
         val EXPLICIT_BACKING_FIELD_IN_INTERFACE by error<KtBackingField>()
         val EXPLICIT_BACKING_FIELD_IN_ABSTRACT_PROPERTY by error<KtBackingField>()
+        val EXPLICIT_BACKING_FIELD_IN_EXTENSION by error<KtBackingField>()
         val ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS by error<KtModifierListOwner>(PositioningStrategy.ABSTRACT_MODIFIER)
         val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS_WARNING by warning<KtProperty>(PositioningStrategy.TYPE_PARAMETERS_LIST)
         val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS by error<KtProperty>(PositioningStrategy.TYPE_PARAMETERS_LIST)
diff --git a/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt b/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt
index a2936a983d5..49dd4ed4e6a 100644
--- a/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt
+++ b/compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt
@@ -486,6 +486,7 @@ object FirErrors {
     val PROPERTY_MUST_HAVE_SETTER by error0<KtProperty>()
     val EXPLICIT_BACKING_FIELD_IN_INTERFACE by error0<KtBackingField>()
     val EXPLICIT_BACKING_FIELD_IN_ABSTRACT_PROPERTY by error0<KtBackingField>()
+    val EXPLICIT_BACKING_FIELD_IN_EXTENSION by error0<KtBackingField>()
     val ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS by error0<KtModifierListOwner>(SourceElementPositioningStrategies.ABSTRACT_MODIFIER)
     val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS_WARNING by warning0<KtProperty>(SourceElementPositioningStrategies.TYPE_PARAMETERS_LIST)
     val LOCAL_VARIABLE_WITH_TYPE_PARAMETERS by error0<KtProperty>(SourceElementPositioningStrategies.TYPE_PARAMETERS_LIST)
diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt
index d1fab8306e2..43304933186 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonDeclarationCheckers.kt
@@ -62,7 +62,7 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
 
     override val backingFieldCheckers: Set<FirBackingFieldChecker>
         get() = setOf(
-            FirExplicitBackingFieldInInterfaceChecker,
+            FirExplicitBackingFieldForbiddenChecker,
         )
 
     override val classCheckers: Set<FirClassChecker>
diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirExplicitBackingFieldInInterfaceChecker.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirExplicitBackingFieldForbiddenChecker.kt
similarity index 83%
rename from compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirExplicitBackingFieldInInterfaceChecker.kt
rename to compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirExplicitBackingFieldForbiddenChecker.kt
index cf32185e54b..2a895cbf38a 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirExplicitBackingFieldInInterfaceChecker.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirExplicitBackingFieldForbiddenChecker.kt
@@ -17,8 +17,9 @@ import org.jetbrains.kotlin.fir.declarations.FirResolvedDeclarationStatus
 import org.jetbrains.kotlin.fir.declarations.impl.FirDefaultPropertyBackingField
 import org.jetbrains.kotlin.fir.declarations.utils.isAbstract
 import org.jetbrains.kotlin.fir.symbols.SymbolInternals
+import org.jetbrains.kotlin.fir.symbols.impl.isExtension
 
-object FirExplicitBackingFieldInInterfaceChecker : FirBackingFieldChecker() {
+object FirExplicitBackingFieldForbiddenChecker : FirBackingFieldChecker() {
     override fun check(declaration: FirBackingField, context: CheckerContext, reporter: DiagnosticReporter) {
         if (declaration is FirDefaultPropertyBackingField) {
             return
@@ -34,10 +35,12 @@ object FirExplicitBackingFieldInInterfaceChecker : FirBackingFieldChecker() {
             return
         }
 
-        val diagnostic = getProperDiagnostic(context)
-
         if (declaration.propertySymbol.isAbstract) {
-            reporter.reportOn(declaration.source, diagnostic, context)
+            reporter.reportOn(declaration.source, getProperDiagnostic(context), context)
+        }
+
+        if (declaration.propertySymbol.isExtension) {
+            reporter.reportOn(declaration.source, FirErrors.EXPLICIT_BACKING_FIELD_IN_EXTENSION, context)
         }
     }
 
diff --git a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt
index 970ae73d914..c49eda0069b 100644
--- a/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt
+++ b/compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt
@@ -173,6 +173,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.OPT_IN_MARKER_CAN
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.OPT_IN_OVERRIDE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.OPT_IN_OVERRIDE_ERROR
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPLICIT_BACKING_FIELD_IN_ABSTRACT_PROPERTY
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPLICIT_BACKING_FIELD_IN_EXTENSION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPLICIT_BACKING_FIELD_IN_INTERFACE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPLICIT_DELEGATION_CALL_REQUIRED
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.EXPOSED_FUNCTION_RETURN_TYPE
@@ -1297,6 +1298,10 @@ class FirDefaultErrorMessages {
                 EXPLICIT_BACKING_FIELD_IN_ABSTRACT_PROPERTY,
                 "Abstract property cannot have a backing field"
             )
+            map.put(
+                EXPLICIT_BACKING_FIELD_IN_EXTENSION,
+                "Extension properties cannot have a backing field"
+            )
             map.put(ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS, "This property cannot be declared abstract")
             map.put(LOCAL_VARIABLE_WITH_TYPE_PARAMETERS_WARNING, "Type parameters for local variables are deprecated")
             map.put(LOCAL_VARIABLE_WITH_TYPE_PARAMETERS, "Local variables are not allowed to have type parameters")
diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
index c9624efc3f5..09f1fcd51a1 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt
@@ -2442,6 +2442,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.EXPLICIT_BACKING_FIELD_IN_EXTENSION) { firDiagnostic ->
+        ExplicitBackingFieldInExtensionImpl(
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS) { firDiagnostic ->
         AbstractPropertyInPrimaryConstructorParametersImpl(
             firDiagnostic as FirPsiDiagnostic,
diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
index b8073f24b4e..0aafa9b8db7 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt
@@ -1727,6 +1727,10 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = ExplicitBackingFieldInAbstractProperty::class
     }
 
+    abstract class ExplicitBackingFieldInExtension : KtFirDiagnostic<KtBackingField>() {
+        override val diagnosticClass get() = ExplicitBackingFieldInExtension::class
+    }
+
     abstract class AbstractPropertyInPrimaryConstructorParameters : KtFirDiagnostic<KtModifierListOwner>() {
         override val diagnosticClass get() = AbstractPropertyInPrimaryConstructorParameters::class
     }
diff --git a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
index fe752821311..b741bb200ae 100644
--- a/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
+++ b/idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt
@@ -2818,6 +2818,13 @@ internal class ExplicitBackingFieldInAbstractPropertyImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class ExplicitBackingFieldInExtensionImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.ExplicitBackingFieldInExtension(), KtAbstractFirDiagnostic<KtBackingField> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class AbstractPropertyInPrimaryConstructorParametersImpl(
     firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,